map band instance (#1392)

scalaz · May 22, 2017 · be8a62f · be8a62f
1 parent 74757ff
commit be8a62f
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diff --git a/core/src/main/scala/scalaz/Map.scala b/core/src/main/scala/scalaz/Map.scala
@@ -1007,6 +1007,10 @@ sealed abstract class ==>>[A, B] {
 }
 
 sealed abstract class MapInstances0 {
+  implicit def mapBand[A, B](implicit A: Order[A], B: Band[B]): Band[A ==>> B] = new Band[A ==>> B] {
+    def append(a: A ==>> B, b: => A ==>> B): A ==>> B =
+      (a unionWith b)(B.append(_, _))
+  }
 
   implicit def scalazMapInstance[S: Order]: Bind[S ==>> ?] with Align[S ==>> ?] with Zip[S ==>> ?] =
     new Bind[S ==>> ?] with Align[S ==>> ?] with Zip[S ==>> ?] {

diff --git a/tests/src/test/scala/scalaz/MapTest.scala b/tests/src/test/scala/scalaz/MapTest.scala
@@ -971,6 +971,7 @@ object MapTest extends SpecLite {
 
   checkAll(order.laws[Int ==>> Int])
   checkAll(monoid.laws[Int ==>> Int])
+  checkAll(band.laws[Int ==>> ISet[Int]])
 
   {
     implicit def equMapConj[A: Equal, B: Equal]: Equal[(A ==>> B) @@ Tags.Conjunction] =